1. Field of the Invention
The present invention generally relates to digital image processing, and more particularly to a method and apparatus for processing a digital image to be displayed on a display device with a backlight module.
2. Description of the Prior Art
For displaying an image on a display device illuminated with a backlight module, such as a liquid crystal display (LCD) panel, the duty (i.e., the illumination strength) of the backlight module is preferred to be adaptively adjusted according to the content to be displayed. One of the purposes is to provide just-fitting illumination for each image to be displayed so as to save electricity power. Accordingly, different images tend to be displayed with different backlight duties in a dynamically controlled backlight system.
With the same image, however, it usually leads to noticeable differences between the case with full duty backlight and the case with adaptively controlled backlight. Such noticeable differences typically arise from contrast loss due to a smaller backlight illumination. Bad visual experience is therefore likely to happen unless the problem is properly handled.
To overcome this problem, it may adjust the Gamma correction factor in the D/A module of the display device, so as to produce a visual effect close to that with full backlight duty. The effect, however, is limited especially for high luminance region of the image, which still suffers from pronounced loss of luminance or contrast.
It may also scale up the luminance of the image to be displayed before it is passed to the D/A module. With scaling up, it means luminance values of all pixels (picture elements) in the image are proportionally increased to compensate the loss of illumination due to dynamically controlled backlight. This produces an effective Gamma correction close to the full backlight duty case in a range broader than above method which adjusts the Gamma correction factor. This method, however, tends to cause saturation in high luminance portion of the image to be displayed. The dynamic range in a brighter scene is thus deteriorated.
In view of foregoing, it can be appreciated that a substantial need exists for methods and apparatus which can advantageously provide a solution to resolve the contrast loss problem such that the power saving adaptive backlight control mode can coexist with a pleasing visual experience.